I use RoundCube, and I'm very happy with it so far. It has a very active developer base, and it offers the closest thing to an email client experience. I'm currently working on an "Outlook" theme for it, which should be done soon.

My mail server stack looks like:

Linode + Postfix + Dovecot (w/mySQL & imap) + RoundCube + SpamAssassin + managesieve.

With this setup, I get more powerful mail filters than Gmail, a better webmail client than most ISPs, and spam controls that I can access directly from my webmail client.

It's not the easiest setup to pull off, though. I'll need to roll out a second one of these before long, and I'll try to write it up next time.

EDIT: Or for $5/month/domain (the same deal I give my clients), I'll give anyone unlimited domain email hosting, unlimited accounts, (practically) unlimited storage.